The use of coated and/or uncoated metal bipolar plates are a viable path because the thin nature of the metal substrate allows for smaller stack designs with reduced weight. Also, the simplicity of stamping a flow field into the metal is a very attractive feature of metal design. From the cost perspective also, metal bipolar plates are relatively inexpensive.
Metal bipolar plates are a subject of corrosion during the fuel cell operation, mainly because of the fluoride ions released as a product of membrane degradation, which makes the implementation of metal plates difficult. Anode plates have shown metal dissolution resulting in the release of Fe, Cr and Ni ions, which impair the proton conductivity of the membrane and can contribute to its chemical degradation. The cathode plate is covered with oxide film, which causes high electrical contact resistance with gas diffusion medium used to distribute the reactive gases to the catalyst layers on MEA. A suppression of aforementioned corrosion and electrical contact resistance processes plays an important role for fuel cell implementation.
Common non-precious corrosion-resistant materials, which are used for protective coatings are Ti, Ta, Nb, Cr, etc. These metal maintain corrosion resistance because of the existence of protective passive layers on their surfaces, however their passive oxides are generally non-conductive oxide films which normally have a high electrical contact resistance with the gas diffusion medium. Moreover, some of these materials undergo severe corrosion under the specific fuel cell operation conditions. Precious noble metal coatings are viable solutions but such coatings are prohibitively expensive.
It is also desirable for the metal bipolar plate materials to have a low water contact angle at the bipolar plate/water border. However, the water contact angles at the precious metal/water interface are not small enough (for example, ΘPtwater˜70°, ΘAuwater˜40°, etc.). The discovery of non-precious corrosion-resistant and highly conductive bipolar plate coatings are desirable for a viable fuel cell for vehicles. Titanium nitride coatings have been disclosed as one of such corrosion resistant plating in U.S. Pat. No. 5,624,769. Whereas titanium nitride is cost effective, stable material under fuel cell operating conditions are also conductive, it does not provide satisfactory protection for bipolar plate material because its film is porous and thus permeable to corroding agents. Also, this coating develops relatively high water affinity, with contact angle close to 60°.